Summary:
The Timmins Talc-Magnesite deposit is a hydrothermally altered ultramafic rock composed, at its core, largely of talc and magnesite although, at its fringes, the content of calcium in the carbonate increases.
The deposit under consideration has long been viewed as a potential source of magnesite and talc. These minerals are found in a variety of deposit types throughout the world and have a variety of end uses.
The best known of the minerals directly and widely exploited for its magnesia content is magnesite (MgCO3), one of the calcite group of rhombohedral carbonates, which includes calcite (CaCO3), siderite (Fe2CO3) and rhodocrosite (MnCO3), among others.
Although the genesis of natural magnesite deposits can be complex, it is distinguished in nature in two distinct physical forms, namely crystalline, (with a wide range of visible crystal sizes) and cryptocrystalline, sometimes referred to as amorphous, where the crystal size is not detectable to the eye and will range from 1 to 10 micrometers. The two types not only differ in crystal structure but in the sizes of the deposits and modes of formations.
Large-scale talc deposits form when magnesium in magnesium-rich rocks reacts with hydrothermal silica in the final phases of regional or contact metamorphism. Most commonly, talc (Mg3Si4O10(OH2)) or steatite (the massive and fine grained form of talc) replaces serpentine in an ultramafic rock like peridotite, either completely or more likely forming an outer rind with zoning (typically granite (silica source), vermiculite, chlorite, actinolite, talc, talc-carbonate and unaltered serpentinite).
The deposit consists of either a single large magnesite-talc altered dunitic komatiite or a series of altered flows. It is hosted by basaltic or andesitic lavas, serpentinized peridotitic komatiite and quartz-carbonate iron formation. It is about 1,800 m long, has a maximum width of 300 m and has been drilled to a depth of 120 m.
The very low CaO content in the magnesite-talc body makes the carbonate mineralization a potential source of refractory magnesia. However, iron substitution in the magnesite lattice means that the iron cannot be removed by standard physical methods. The iron, therefore, limits the grade of magnesia concentrate or dead-burned refractory product.
Several occurrences of talc-magnesite are known to be present on the property, the largest of which is located to the south of the diabase dike and is referred to as the A Zone. This zone has been traced by surface trenching, mapping and drill hole information along a strike length of approximately 1,000 m, to depths of approximately 100 to 150 m and achieves widths of 200 m at surface. The information available to date suggests that the A Zone has a near vertical dip in an overall sense, although the north and south contacts can be seen to locally dip steeply to either the north or south. A second zone of magnesite mineralization is located to the north of the diabase dike, although its dimensions and extents are known only from a small number of drill holes that suggest a strike length on the order of 1,000 m, with widths measuring on the order of a few tens of metres. A third zone of mineralization is located in the southwestern portions of the claim holdings and is exposed in surface outcroppings, but the extents of this zone are not known in detail.
The core magnesite zone is a massive, coarse grained, over-printed and re-crystallized magnesite and lesser talc unit showing no visible relic original textures. Within surfacestripped zones the exposures show a well developed set of quartz-carbonate extensional veins and stockworks, with subvertical to steep south dipping linker veins that strike easterly and are sigmoidally curved, moderately dipping tension ladder structures. Drilling indicates that the “high-grade” magnesite zones are wider than when exposed on surface, and carry much less veining than anticipated.
The transition zone has been logged as a talc carbonate-chlorite zone. It is physically similar to the above described core magnesite zone, other than it tends to be darker in tone (medium grey) due to the presence of aphanitic to fine grained black chlorite and tends overall to be more bladed to foliated in texture. The zone may be richer in talc and has a strongly developed carbonate groundmass, but shows variable lesser amounts of magnesite in inverse proportion to developed ferro-dolomite.